1. Field of the Invention
The present invention is in the field of coherent fiber optic systems that detect the phase difference between two interfering light waves in sensing applications and in telecommunications applications.
2. Description of the Related Art
In coherent fiber-optic systems for sensing and telecommunications applications, there is a well known signal fading problem that results in low sensitivity and instability. Briefly, such systems operate by detecting the intensity of an optical output signal produced by the interference of two optical waves travelling in two optical propagation paths. For example, in rotation sensors comprising a loop of optical fiber, two counterpropagating optical signals are combined such that they interfere and produce an optical output signal having an intensity that varies in accordance with the rotation rate of the loop. The intensity of the combined optical output signal depends upon the relative phase difference between the two optical waves. The optical phase difference is a result of phase biasing caused, for example, by the parameter to be sensed (e.g., rotation) in a sensing application, or by signal information in a telecommunications application. The optical phase difference is further sensitive to environmental parameters that introduce additional phase differences. This problem is particularly acute when the phase difference between the two interfering waves is very close to an integer multiple of .pi. (i.e., N.pi., where N is 0, 1, 2, 3, etc.)
Several approaches have been devised to overcome the above-described signal fading problem. For example, D. A. Jackson, et al., "Elimination of drift in a single-mode optical fiber interferometer using a piezoelectrically stretched coiled fiber," APPLIED OPTICS, Vol. 19, No. 17, 1 September 1980, pp. 2926-2929, describes a servo driven piezoelectrically stretched coiled fiber for maintaining a single-mode optical fiber interferometer in quadrature to actively stabilize the phase bias. James H. Cole, et al., "Synthetic-Heterodyne Interferometric Demodulation," IEEE JOURNAL OF QUANTUM ELECTRONICS, Vol. QE-18, No. 4, April 1982, pp. 694-697, and B. Y. Kim, et al., "Phase-reading, all-fiber-optic gyroscope," OPTICS LETTERS, Vol. 9, No. 8, August 1984, pp. 378-380, describe systems that use heterodyning to overcome the signal fading problem. Th. Niemeier, et al., "Quadrature outputs from fiber interferometer with 4.times.4 coupler," OPTICS LETTERS, Vol. 11, No. 10, October 1986, pp. 677-679; Sang K. Sheem, "Fiber-optic gyroscope with [3.times.3] directional coupler," Applied Physics Letters, Vol. 37, No. 10, 15 November 1980, pp. 869-871; and David W. Stowe, et al., "Demodulation of Interferometric Sensors Using a Fiber-Optic Passive Quadrature Demodulator," JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. LT-1, No. 3, September 1983, pp. 519-523, describe passive devices that produce quadrature phase information on the optical output signals. For example, in the Stowe, et al., article, two interferometers are constructed such that when one interferometer is operating at its minimum sensitivity, the other interferometer is operating at its maximum sensitivity. The present invention provides a new approach to the generation of signals that provide quadrature phase information to overcome the signal fading problem.